The invention relates to a scanning optical microscope comprising a radiation source unit for supplying a radiation beam and an objective system for focussing the radiation beam to a scanning spot in an object plane. At least one beam-deflecting element is arranged in the radiation path of the radiation beam for displacing the scanning spot in the object plane and a beam splitter is arranged in the radiation path of the radiation beam between the beam-deflecting element and the objective system for splitting off a reference beam from the radiation beam. A radiation-sensitive detection system in which a reference plane is present is arranged in the radiation path of the reference beam for detecting the position of a reference spot formed by the reference beam in the reference plane. A microscope of this type may be used for sampling a biological preparation or a piece of semiconductor material which has undergone one or more process steps for accommodating an integrated circuit therein.
A scanning optical microscope of the type described in the opening paragraph is known from the Article "High-precision laser scanning microscope" in IBM Technical Disclosure Bulletin, Vol. 28, no. 10, pages 4260-4261 (March 1986). In the microscope described in this Article the scanning spot is moved on the surface of an object by means of a rotatable mirror. A diffraction grating, with which two reference beams are separated from the scanning beam, is arranged in the radiation beam. Each reference beam is focussed in a reference plane in which a series of apertures is provided. Radiation from the reference beams, which passes the reference plane through the apertures, is used to generate a localisation signal for the scanning spot in the object plane.
In the known microscope the apertures in the reference plane and the size of the reference spot are very small. Dust and other contaminations on the reference plane therefore considerably affect the aperture radiation transmission and thus the accuracy with which the scanning spot position can be established.